s
Medium Voltage Technology
Switchgear Application Guide
Medium Voltage Technology
Switchgear Application Guide
A
Selection of Switching Devices
B
Switching Duties in MV networks Î Overview on page 20
C
Characteristic Values of Switching Devices
D
Selection of HRC Fuses
E
Selection of Surge Arresters
F Switchgear
Configuration
G
Influences and Stress Variables
H
Selecting and Rating Switchgear Î Overview on page 90
I Annexes
J Standards
Dipl. Ing. Ansgar Müller Siemens AG Energy Sector E D MV ST P.O. Box 3220 91050 Erlangen Germany E-Mail: [email protected] Edition 12E3 • 2011-07
C
ONTENTS
A
Selection of the Appropriate Device ... 11
A 1 Selection criteria ...11
A 2 Suitability under normal operating conditions...12
A 3 Suitability under fault conditions...12
A 4 Switching frequency and endurance classes ...13
A 4.1 Circuit-breaker ...13
A 4.2 Switches and switch-disconnectors...15
A 4.3 Contactos...16
A 4.4 Disconnectors and earthing-switches...17
A 4.5 Three-position switching devices...17
A 4.6 Guidance for use ...18
A 5 Aspects of selection for disconnectors...19
B
Switching Duties in Medium Voltage Networks ... 20
B 1 Overview...20
B 2 Distribution transformers ...22
B 3 Unit transformer...22
B 4 Petersen (arc suppression) coil...23
B 4.1 Overvoltage protection...23
B 4.2 Selection of surge arresters ...23
B 5 Cable or overhead line ...24
B 6 Cable or line with short-circuit limiting reactor...24
B 7 Compensation coils (shunt reactors) ...26
B 8 Motors ...26
B 8.1 Overvoltage protection for motors...27
B 8.2 Direct connected motor (direct on line) ...27
B 8.3 Motor with transformer (unit)...28
B 8.4 Motor with starting transformer...29
B 8.5 Motors with starting converter...30
B 8.6 Motors with their own p.f. improvement ...31
B 9 Generators ...32
B 9.1 Selection of the circuit-breaker ...32
B 9.2 Overvoltage protection...33
B 9.3 Generators with Ik" ≤ 600 A feeding into a cable system ...33
B 9.4 Generator-transformer unit ...33
B 9.5 Generators connected to the HV grid...35
B 10 Furnace transformer ...35
B 10.1 Insulation co-ordination for operating voltages above 36 kV...36
B 11 Converter transformer ...37
B 11.1 Protective measures...37
B 11.2 Systems up to 15 kV operating voltage ...38
B 11.3 Systems above 15 kV operating voltage ...38
B 12 Capacitor banks and filter circuits ...39
B 12.1 Capacitor banks (without reactor)...39
B 12.2 Reactor-capacitor combinations...39
B 12.3 Permissible inrush current...40
B 12.4 Measures to limit the inrush making current ...41
B 12.5 Permissible voltage at the circuit-breaker...43
B 12.6 Determination of the voltage on the load side of the breaker ...44
B 12.7 Check for permissible stress on the circuit-breaker ...45
B 13 Audio frequency ripple control systems ...46
C
Rated Values of Switching Devices ... 47
C 1 Overview...47
C 2 Rated voltage Ur...48
C 3 Rated insulation level Ud and Up...48
C 4 Rated normal (operating) current Ir...48
C 5 Rated short time withstand current Ik...49
C 6 Rated duration of short circuit tk...49
C 7 Rated peak withstand current Ip...49
C 8 Rated short-circuit making current Ima...49
C 9 Rated short-circuit breaking current Isc...50
C 10 Rated mainly active load-breaking current I1...50
C 11 Rated closed-loop breaking current I2a, I2b...50
C 12 Rated no-load transformer breaking current I3...50
C 13 Rated cable-charging breaking current I4a...50
C 14 Rated line-charging breaking current I4b...50
C 15 Rated earth fault breaking current I6a...50
C 16 Rated cable- and line-charging breaking current under earth fault conditionsI6b...51
C 17 Cable switching current under earth fault conditions with superimposed load current...51
C 18 Rated transfer current Itransfer...51
C 19 Rated voltage UL (surge arrester with spark gaps)...51
C 20 Rated voltage Ur (metal oxide surge arrester)...51
C 21 Continuous operating voltage Uc (metal oxide surge arrester) ...51
C 22 Sparkover voltage ...52
C 23 Residual voltage ures...52
D
Short-Circuit Protection with HRC Fuses ... 53
D 1 Selection criteria ...53
D 2 HRC fuses for distribution transformers...53
D 2.1 Fuse rated current IrHH...53
D 2.2 Rated voltage UrHH...55
D 3 HRC fuses for capacitors ...56
D 3.1 Fuse rated current IrHH...56
D 3.2 Rated voltage UrHH...56
D 4 HRC fuses for motors ...56
D 4.1 Fuse rated current IrHH...56
D 4.2 Rated voltage UrHH...58
D 5 Co-ordination with a switch or contactor...59
D 6 Let-through (cut off) current ...62
D 7 Heat losses...63
E
Selection of Overvoltage Protection Devices... 64
E 1 Selection criteria ...64
E 2 Continuous operating voltage and rated voltage...64
E 3 Rated discharge current...65
E 4 Residual and sparkover voltage (protection level)...66
E 4.1 Switching overvoltages...66
E 4.2 3 arresters in line-earth connection ...67
E 4.3 6 arrester format ...67
E 4.4 Lightning overvoltages ...68
E 5 Energy absorbtion capability ...68
E 6 Short-circuit withstand and housing ...69
E 7 Switchgear with overhead line connections...69
F
Switchgear Configuration... 71
F 1 Principles of configuration...71
F 1.1 General...71
F 1.2 Requirements and complementary characteristics...71
F 1.3 Procedure ...72
F 2 Switchgear requirements...73
G
Influences and Stress Variables ... 75
G 1 Network characteristic values ...75
G 1.1 Line voltage...75
G 1.2 Short-circuit current ...75
G 1.3 Normal current and load flow ...76
G 1.4 Neutral point earthing ...76
G 1.5 Underground / overhead lines ...77
G 1.6 Overvoltage protection...77
G 1.7 Power quality (unstable loads)...78
G 2 Line Protection, measurement and metering...78
G 2.1 Short-circuit protection ...78
G 2.2 Protection functions ...79
G 2.3 Tripping times...79
G 2.4 Measurement and metering...79
G 2.5 Redundancy...80 G 3 Infeed types...80 G 4 Operating sites...81 G 4.1 Installation location...81 G 4.2 Accessibility...81 G 4.3 Switchgear room ...82 G 4.4 Buildings ...82
G 4.5 Transportation and assembly ...83
G 5 Environmental conditions ...83
G 5.1 Ambient room conditions...83
G 5.2 Altitudes above 1000 m ...84
G 5.3 Ambient temperature and humidity ...84
G 6 Industry-specific application...85
G 6.1 Switching duty and capacity ...85
G 6.2 Switching frequency of the loads...85
G 6.3 Frequency of switchover between busbars ...86
G 6.4 Availability (faults, redundancy, switchover time)...86
G 6.5 Modifications or extensions ...87
G 7 Operating procedures ...87
G 7.1 Operation...87
G 7.2 Work activities ...88
G 7.3 Inspection and maintenance ...89
H
Selecting and Rating Switchgear... 90
H 1 Overview (Checklist) ...90
H 2 Primary rated values...93
H 2.1 Rated voltage Ur...93
H 2.2 Rated insulation level...94
H 2.3 Rated short-circuit currents...96
H 2.4 Rated duration of short-circuit tk...97
H 2.5 Rated normal current Ir...98
H 3 Busbar arrangement ...99
H 3.1 Single busbars ...99
H 3.2 Double busbars...99
H 3.3 Design of double busbars...100
H 3.4 Operating principle of bus couplings...100
H 4 Switching Devices ...101
H 5 Switchgear design and panel type...102
H 5.1 Basic selection criteria ...102
H 5.2 Panel and block design...103
H 5.3 Installation conditions, transportation and assembly ...104
H 5.4 Additional aspects...104
H 6 Electrical and mechanical reserves ...105
H 7 Insulation medium...106
H 8 Design of the isolating distance ...107
H 8.1 Accessibility...107
H 8.2 Voltage tests on cables...108
H 8.3 Switching frequency ...108
H 9 Enclosure...109
H 9.1 Degree of protection...109
H 9.2 Internal arc classification ...110
H 9.3 Pressure absorbers and pressure relief ducts...110
H 10 Compartments ...111
H 10.1 General selection criteria ...111
H 10.2 Loss of service continuity category ...111
H 10.3 Access control...113
H 11 Feeder circuit components ...113
H 11.1 Cable connections ...114
H 11.2 Instrument transformers ...114
H 11.3 Earthing switches ...114
H 11.4 Surge arresters...114
H 12 Busbar / metering panel components ...115
H 13 Electronic secondary equipment ...116
H 13.1 Electromagnetic compatibility (EMC)...116
H 13.2 EMC requirements on the switchgear...116
H 13.3 EMC requirements on electronic equipment ...117
I
Appendix... 119
I 1 Damping relaxation oscillations...119
I 1.1 Relaxation oscillations (ferroresonance)...119
I 1.2 Standard values for damping resistors ...120
I 1.3 Calculating the damping resistance...120
I 1.4 Formula, symbols and indices...120
I 1.5 Standard, uninterrupted line operation...120
I 1.6 Operation with earth fault ...121
I 1.7 Damping resistance...122
I 1.8 Sample calculations...122
I 2 RC-circuitry for protection of MV equipment...123
I 3 Installation of overvoltage protection devices ...124
I 4 Overvoltage factors (Definition)...125
I 5 Abbreviations, Symbols and Formula Variables ...126
I 6 Diagram Symbols...127
J
Relevant Standards and Regulations... 128
J 1 Statutory regulations for medium-voltage equipment ...128
J 2 Generic standards for switching devices and switchgear...129
J 3 Product standards for switching devices...130
J 4 Product standards for switchgear and accessories ...131
A
S
ELECTION OF THE
A
PPROPRIATE
D
EVICE
A 1
Selection criteria
The effective selection of devices for a particular switching duty is determined by three main re-quirements:
a) the operational current switching capability b) the fault current switching capability c) the frequency of switching
The section below deals with the criteria for following devices:
• Circuit breakers • Vacuum contactors • HRC fuses • Vacuum switches • SF6 switch disconnectors
• Hard gas (gas evolving) and airbreak switches and switch disconnectors
The most important of these is the required switching capability. If more than one device fulfils these requirements a) and b), the frequency of switching might be the critical factor. Individual equipments differ in the number of mechanical and electrical operations for which they are de-signed, the length of time between maintenance and the cost and inconvenience of that mainte-nance.
Additional criteria may be:
• the voltage withstand level of the gap;
in switchboards where the switches are not withdrawable, devices which ensure a safe isolation gap are needed. Switch disconnectors fulfil the safety gap requirements, switches, circuit break-ers and contactors do not. They need an additional disconnector or similar device in series. In switchboards with withdrawable or truck-mounted equipment this is unimportant, the gap is es-tablished by the act of withdrawal.
• the drive mechanism;
for duties such as synchronising and (multiple-) auto-reclose, a drive mechanism with defined, short closing and opening times. Only stored energy systems suffice; springs which have to be charged first are unsuitable.
Circuit breakers can switch on and off (make or break) all values of current within their rated
ca-pability, from small inductive or capacitive load currents up to full short circuit currents, and under all the fault conditions like earth fault, phase displacement (out-of-phase switching) etc.
Switches can switch on and off operating currents up to their rated interrupting capability and can
close onto existing short circuits up to their rated fault making current. They have very limited fault current breaking capability only.
Switch disconnectors combine the functions of switches and disconnectors or, put another way,
they are switches with the specific safety gap required of disconnectors.
Contactors are load switching devices with limited short circuit making and breaking capacity.
They are electrically operated and are used for high switching rates, e.g. for motor control.
Fuses (or, more accurately, the fuse link) provides a single interruption of a short circuit current.
A 2
Suitability under normal operating conditions
Switch and switch-disconnector No. Operating duty
Circuit-breaker Vacuum SF6 Air-, Hard gas
Vacuum contactor 1 Transformer
(Star point transformer) X X X X X 2 Converter transformer X 3 Furnace transformer X X 4 Petersen coil X X X 5 Compensation coil X 6 Motor X X ( X ) X 7 Generator X X ( X )
8 Cable, Ring mains X X X X
9 Overhead lines X X X X
10 Single capacitor X X X ( X ) X 11 Paralleling of
capacitors X X X
12 Filter circuit X X
13 Ripple control circuit X X X 14 Synchronising X
X = suitable and effective use (X) = limitedly suitable
Table A-1: Switching capability under normal conditions
A 3
Suitability under fault conditions
Using the following three tables, check the fault currents the device must be able to switch.
Switch and switch-disconnector No. Fault duty
Circuit-breaker Vacuum SF6 Air-, Hard gas
Vacuum contactor 1 Closing onto fault X X X X ( 1 ) 2 Terminal fault X ( 2 ) ( 2 ) ( 2 ) ( 2 ) 3 Auto-reclose X
4 5 6
Fault on load side of - Generator - Reactor - Transformer X X X ( 2 ) ( 2 ) ( 2 ) ( 2 ) 7 Locked rotor motor X X ( 2 ) X 8 Double earth fault X ( 2 ) ( 2 ) ( 2 ) ( 2 ) 9 Phase opposition X
(1) Without HRC fuse, only limited fault switching capability. (2) Only as switch-fuse combination; current interruption by the fuse.
Switch and switch-disconnector No. Fault duty
Circuit-breaker Vacuum SF6 Air-, Hard gas
Vacuum contactor 10 Unloaded cable, OHL
fault on network side
X X X ( 1 ) X
11 Loaded cable, OHL fault on network side
X X X X
12 Unloaded cable, OHL fault on load side
X X X X X
13 Loaded cable, OHL fault on load side
X X X ( 1 ) X
(1) for small currents only
Table A-3: Switching under earth fault conditions
Switch and switch-disconnector No. Fault duty
Circuit-breaker Vacuum SF6 Air-, Hard gas
Vacuum contactor 14 Safe disconnection (Disconnection under load) X X 15 Rapid changeover X 16 Transformer with short-circuited winding X X ( 1 ) ( 1 ) ( 1 ) (1) Only as switch-fuse combination; current interruption by the fuse.
Table A-4: Other fault conditions
A 4
Switching frequency and endurance classes
When a range of devices fulfils the electrical requirements, the frequency of switching in connec-tion with the endurance can be an addiconnec-tional selecconnec-tion criterion. The standards distinguish between mechanical and electrical endurance, which are also applicable to equipment "mixed"; e.g. a switch can mechanically correspond to the class M1 and electrically to the class E3.
The following tables show the endurance classes of the switching devices and give a recommenda-tion about useful use with that.
A 4.1
Circuit-breaker
A 4.1 a Mechanical and electrical endurance – classes M and E
IEC 62271-100 [7] defines the mechanical endurance by a certain number of operating cycles (class M), the electrical endurance, however, merely with the verbal description “normal” and “extended” endurance.
To the orientation, what “normal” and “extended electrical endurance” means, the grey shaded table elements indicate the operating cycles which average modern vacuum circuit-breakers can perform.
The numbers for short circuit operation (Isc) are derived from the operating sequences of the type test. As a rule these are minimum numbers; actually, vacuum circuit-breakers of the class E2 with auto-reclosing capability, which are used in electricity grids with overhead lines, may break the smaller short-circuit currents, usual there, several hundred times.
Furthermore it is worth mentioning, that almost all modern vacuum circuit-breakers can switch the rated normal current with the number of the mechanical operating cycles.
Class Description
M1 2.000 Operations normal mechanical endurance M
M2 10.000 Operations extended mechanical endurance, limited maintenance E1 * I r 2.000 Operations * I sc 6 x Open 4 x Close
normal electrical endurance
(circuit-breaker, not falling into class E2)
* I r 10.000 Operations * I sc 6 x Open 4 x Close without auto-reclosing duty E E2 * Ir 10.000 Operations * I sc 50 x Open 15 x Close with auto-reclosing duty
extended electrical endurance without maintenance of interrupting parts
* Derived numerical values to the orientation = > see text.
Table A-5: Classes M and E of circuit-breakers
A 4.1 b Capacitive current switching – class C
Class C defines the capacitive current breaking performance comprising the characteristics of three switching duties, i.e. the closing and switching off of lines, cables and capacitor banks (single and back-to-back capacitor banks).
Class Peformance on breaking of capacitive currents
C1 24 x O per 10…40% Ilc, Icc, Ibc
24 x CO per 10…40% Ilc, Icc, Ibc Low probability of restrikes
C C2 24 x O per 10…40% Ilc, Icc, Ibc 128 x CO per 10…40% Ilc, Icc, Ibc Very low probability of restrikes Restrike-free breaking operations at 2 of 3 test duties
Table A-6: Classes for capacitive current switching
The selection of the class depends on the operating conditions, the switching frequency and the pos-sible effects of restrikes.
- Class C1 is suitable for infrequent switching of transmission lines and cables;
- Class C2 is recommended for capacitor banks as well as for frequent switching of transmission
A 4.1 c Cable or line system – class S
Class S defines the type of the grid where the circuit-breaker is intended to be employed. The switching duties differ in magnitude and rate of rise of the the transient recovery voltage (TRV) while breaking short-ciruit currents. In line systems the TRV and the associated dielectric stress on the contact gap can be much higher due to the smaller phase-to-earth capacitances.
Circuit-breakers of indoor switchgear are always to be assigned to class S1, i.e. cable system. The same applies to feeders where an overhead line is connected to the switchgear via a cable. Class S2 is virtually not relevant to metal-enclosed medium voltage switchgear. In terms of the standard a circuit-breaker in an overhead line system is, for example, a (outdoor) breaker directly connected to the line without cable.
Class Description
S1 Circuit-breaker intended to be used in a cable system S S2 Circuit-breaker used in a line-system, or
in a cable-system with direct connection (without cable) to overhead lines
Table A-7: Classes for cable or line system system
A 4.2
Switches and switch-disconnectors
IEC 62271-103 [8] specifies classes for so-called general purpose switches. In addition there are „limited purpose“ and „special purpose“ switches1. General purpose switches – as the name sug-gests – have to switch different kinds of operating currents: load currents, closed-loop currents, transformer magnetising currents, cable and line charging currents as well as making short-circuit currents. General purpose switches intended for use in isolated neutral point systems or in systems earthed by a high impedance shall be capable of switching under earth fault conditions. This versa-tility is reflected in the relatively comprehensive definition of the classes, which are applicable for
• Vacuum switches
• SF6 switches and switch-disconnectors
• Air and hard-gas switches and switch-disconnectors
For switch-disconnectors the table details apply to the function “switch”. See also section A 4.5 Three-position switching devices.
1 Limited purpose switches need only cope with a certain range of the performance of a general purpose switch. Special purpose switches are used for selected duties such as switching of single capacitor banks, paralleling of capacitor banks, closed-loop circuits built up by transformers in parallel, or motors (under steady-state and stalled conditions).
Class Switching cycles Description
M1 1000 Mechanical endurance M
M2 5000 Increased mechanical endurance E1 10 x Iload 10 x Iloop 2 x Ima E2 30 x Iload 20 x Iloop 3 x Ima E E3 100 x Iload 20 x Iloop 5 x Ima 20 x 0.05 ⋅ Iload 10 x Icc 10 x 0.2 to 0.4 ⋅ Icc 10 x Ilc 10 x Ief1 10 x Ief2 C1 Restrikes permitted
(number not defined) C C2 10 x Icc 10 x Ilc 10 x Isc 10 x Ibb additionally each 10x 0,1 …0,4 x Icc, Isb, Ibb No restrikes
Test currents: (old) Iload active load-breaking current I1
Iloop closed-loop breaking current I2a
Icc cable-charging breaking current I4a
Ilc line-charging breaking current I4b
Isb capacitor bank breaking current I4c
Ibb back-to-back capacitor bank I4d
breaking current
Ief1 earth fault breaking current I6a
Ief2 cable- and line-charging breaking I6b
current under earth fault conditions Ima Short-circuit making current Ima
Table A-8: Classes for switches
A 4.3
Contactos
The standard for contactors has not yet defined endurance classes. Customary designs feature me-chanical and electrical endurance in the order of 250,000 and 1,000,000 operation cycles. The can be encountered where extremely high switching rates occur; e.g. > 1 / hour.
However, classes for the suitability of breaking capacitive currents are defined.
Class Description
C0 Not defined ≤ 1 restrike per interruption
C1
24 x O 24 x CO
each per 10…40% Ilc, Icc, Ibc Low probability
of restrikes ≤ 5 cummulated restrikes on test duties BC1 and BC2 C
C2 24 x O per 10…40% Ilc, Icc, Ibc
128 x CO per 10…40% Ilc, Icc, Ibc
Very low probability
of restrikes No restrikes 2
Table A-9: Classes for contactors
- Contactors of C2 class are suitable for capacitor banks.
A 4.4
Disconnectors and earthing-switches
IEC 62271-102 [21] defines the classes for disconnectors and earthing-switches. Since disconnec-tors have no switching capacity3, only classes fort he mechanical endurance are specified.
Class Operations Description
M0 1000 for general duties M1 2000
M
M2 10.000 extended mechanical endurance
Table A-10: Classes for disconnectors
The classes for earthing switches specify the short-circuit making capability (earthing in case of voltage still being present). E0 designates a normal earthing switch, whereas E1 and E2 correspond to earthing switches with short-circuit making capability; so-called make-proof earthing switches.
Class Operations Description
E0 0 x Ima no short-circuit making capability
E1 2 x Ima
for general duties E
E2 5 x Ima
short-circuit making capability
reduced maintenance
Table A-11: Classes for earthing-switches
A 4.5
Three-position switching devices
Three-position switching devices combine two or three functions within one device
• Disconnecting
• Earthing and short-circuiting
• Load switching
• Interrupting short-circuits
For three-position switching devices classes are designated for each individual function, i.e. as if there were two or three separate switching devices.
3 Disconnectors up to 52 kV rated voltage can switch off only “negligible” currents up to 500 mA (for example voltage transformers), or higher currents only if no significant change in voltage occurs (for example busbar transfer when bus coupler is closed).
A 4.6
Guidance for use
Table A-12 shows the average lives of switching devices and gives a recommendation for appropri-ate usage. The table gives only guide values; they can be taken into account when no other criterion is of greater importance.
Mechanical operations
Operations at rated … Device
Nos. Class normal current
breaking current
Class
Switching frequency which gives reason-able lifetime
Circuit-breaker 10 000 M2 10 000 5 - 400 E2 no criterion Vacuum switch 5 000 M2 5 000 10 E3 1 / week SF6 switch 1 000 M1 100 30 5 E2 E3 ≤ 1 / month Hard-gas switch 1 000 M1 10 30 2 E1 E2 ≤ 1 / year Vacuum contactor 106 not
defined 106 1 000 defined not > 1 / hour
Table A-12: Endurance of switching devices
The column “breaking current” gives only average numbers of operations. The actual lifetime can be much higher since the full rated value rarely occurs in practice. For circuit-breakers the lower value refers to high short-circuit currents (≥ 50 kA), the upper value to small currents (12.5 kA). For switches the column mentions the transfer current (Itransfer) of switch-fuse combinations [30]. With vacuum contactors the value refers to the limit breaking capacity. The class designation refers to the corresponding product standard.
Regarding switching frequency circuit-breakers are an exception: The switching rate is not a decid-ing factor for the choice because circuit-breakers are used, if short-circuit breakdecid-ing capacity is re-quired.
A 5
Aspects of selection for disconnectors
For work on switchboard components, it must always be possible to establish a safe disconnection gap. For this, a switching device or an arrangement of equal value is essential, also an interlock be-tween this disconnector and the load, power or earthing switch. With combined devices like switch-disconnectors or three-position devices, an additional disconnector is unnecessary and even separate interlocking may become superfluous.
The average mechanical endurance of disconnectors (truck or withdrawable part) amounts 1000 or 2000 operations, corresponding to the classes M0 or M1. This is completely sufficient for most of the applications.
The life of the disconnector in double busbar switchboards plays an important role: In some net-works, for operational reasons, it is necessary to switch frequently from one busbar to the other. Because of the limited life of conventional disconnectors in comparison to the main switching de-vices, not all switchboards are suitable for this application.
B
S
WITCHING
D
UTIES IN
M
EDIUM
V
OLTAGE
N
ETWORKS
B 1
Overview
The table gives an overview whether special measures have to be taken on selection of switching devices. Details are described in the chapters listed in the right column.
Switching duty Advice Section
Distribution transformers 4
Power transformers
If the transformer is fed by a switch-fuse combina-tion, the fuses must be selected according to the rules for transformer circuits and, in order to pro-tect the switch, according to the IEC standard for switch-fuse combinations.
For circuit-breakers no special requirements apply in this respect.
B 2, D 2 and D 5 - D 7
Unit transformer See • Motor
• Generator • Furnace transformer • Converter transformer B 8 B 9 B 10 B 11
Petersen coil Surge arrester or limiter B 4
Overhead line or cable Switches and switch-disconnectors are very limited in suitability for earth fault location (by selective switching of circuits): only up to their switching capability under earth fault conditions - with or without load (see manufacturer data)!
B 5
Cable or overhead line with short-circuit limiting reactor
Surge arresters are required at the “short” end of the cable / line
B 6
Compensation coil (shunt reactor)
Overvoltage protection is required generally
- RC-circuits
- if current < 600 A, additionally surge arresters - protection at the busbar, depending on the
net-work configuration
B 7
Motor
(see next page)
Overvoltage protection is required if motor starting
current is < 600 A B 8
4 Distribution transformers (rated power uo to 2.5 MVA) transform energy from a primary to secondary distribution or industrial network. Equipment connected downstream a distribution transformer have its own switchgear for switch-ing under normal service conditions.
Switching duty Advice Section
Motor
(continued)
Peak and short-circuit making current: due to re-verse power feed with large motors or some motors connected, the peak current rating of the switch-gear may exceed the standardized value of 2.5 Ik (or 2.6 Ik at 60 Hz).
Motor with HRC fuse: select the fuse according to starting current and time of the motor!
D 4 + D 5 to D 7 Generator Overvoltage protection is necessary, if short-circuit
current of generator is < 600 A
Peak and short-circuit making current: with large machines the peak current may exceed the stan-dardized value of 2.5 Ik (or 2.6 Ik at 60 Hz) – the critical share is the peak current from the network side.
Rated voltage: Consider temporary overvoltage due to the effect of load shedding.
Rated short-circuit breaking current: Consider the DC component of the generator current.
B 9
Furnace transformer Protection by RC-circuits and surge arresters is required, individually matched to the system (in most cases also at the busbar)
B 10
Converter transformer Surge arresters B 11
Capacitor bank
Back-to-back switching of capacitor banks
Inrush current: consider the maximum permissible closing current of the circuit-breaker; where appli-cable, use damping measures.
Rated normal current: consider additional heating due to harmonic currents.
Rated voltage: consider the increased voltage if capacitor bank is fitted with limiting reactors HRC fuses: select rated voltage and rated current 1-2 steps higher
B 12
D 3
Filter circuit Rated voltage: Consider operating voltage limits at
filter capacitor (see capacitor banks) B 12 Audio frequency
ripple control system
– – – B 13
B 2
Distribution transformers
This covers all transformers in industrial and power utility networks, with exception of the special transformers in section B 2.
Duty: Unloaded transformer Loaded transformer
Current: - Magnetising current 1 to 3 % of rated
transformer current (IrT)
- Inrush current up to 15 IrT
up to 120 % of rated trans-former current
cos φ: < 0.3 induktive
< 0.15 during inrush 0.7 to 1.0 ind. Remarks: - Protection relays shall have inrush restraint
- Switching device shall have low chopping
current
– – –
Table B-2: Switching of distribution transformers
Transformer during inrush: when switching off during the inrush phase, currents up to 15 times the rated current at cos ϕ = 0.15 may occur, heavily superimposed by harmonic currents. Air or hard-gas switches are not capable of interrupting those currents. The transformer protection relay must feature an inrush restraint in order to avoid switching off of the transformer during the inrush. Where HRC fuses are used for short-circuit potection, the selection may be done in accordance with the procedure defined in IEC 60787 [27]; the corresponding German standard VDE 0670-402 [28] presents a table of fuse ratings allocated to transformer ratings – see table 2 therein. Additionally – where applicable – the requirements of switch-fuse combinations to IEC 62271-105 [30] must be taken into account. See section D 5 for the procedure to configure these combinations.
B 3
Unit transformer
Transformers in this arrangement normally feed only one, special load. The switching duty is then determined by the characteristics of that load. For further clarification see:
• Motor (with transformer, starting transformer) → Section B 8
• Generator → Section B 9
• Furnace transformer → Section B 10
B 4
Petersen (arc suppression) coil
Petersen coils in earth fault compensated, normally unearthed, networks earth the network at the starpoint of either a transformer or an earthing transformer. When an earth fault occurs, the coil is switched in to produce a purely inductive current which should compensate (equal) the capacitive earth fault current which is flowing into the fault. If the Petersen coil is switched off during the fault, multiple re-ignitions can cause overvoltages. Surge arresters will protect against these. Switching: without earth fault in network with earth fault in network
Current: approx. 1% of transformer rated current
up to 300 A
cos φ: less than 0.15 ind. less than 0.15 ind.
Remarks: as unloaded transformer overvoltages due to multiple re-ignition possible
Table B-3: Switching of arc suppression coil
B 4.1
Overvoltage protection
Arresters at the transformer or neutral point earthing transformer terminals in phase-earth connec-tion, or parallel to Petersen coil; if the coil can be switched directly (left figure), the arrester has to be installed there.
Figure B-1: Protection methods for arc suppression coils
B 4.2
Selection of surge arresters
It is essential that the transformer insulation corresponds to the upper standard value of the rated insulation level in accordance with table 2, IEC 60076-3 [6]; these insulation ratings equal the val-ues in column (4), Table H-3. Otherwise the arrester can be selected as described in section E.
Surge arrester Rated voltage Ur Continuous operating voltage Uc At the primary terminals of the
(neutral earthing) transformer Ur ≥ 1.25 · Uc Uc ≥ Um Parallel to Petersen coil Ur ≥ 0.8 · Um Uc ≥ Um / √3 Umax Maximum operating voltage of the power system
Table B-4: Selection of surge arresters for an arc-suppression coil
B 5
Cable or overhead line
This refers to unloaded cables and lines. In this case, only the capacitive charging current flows. See section B 6 for cable and lines with short-circuit limiting reactor.
Switching: Cable Overhead line
Current: up to 100 A up to 10 A
cos φ: capacitive capacitive
Remarks: Vacuum switches are restrike-free
Switches and switch-disconnectors are very limited in suitability for earth fault location (by selective switching of circuits): only up to their switching capabil-ity under earth fault conditions - with or without load (see manufacturer data)!
Table B-5: Switching of cables or overhead lines
Additional measures: None
B 6
Cable or line with short-circuit limiting reactor
If two switchgear installations with different peak and short-time current ratings are connected, a reactor must limit the short-circuit current to the level of the installation with the lower rating. On energising the connection between installation A and B under no-load condition impermissible overvoltages may occur if the circuit-breaker in installation A closes the “long” cable (some hun-dred metres) while the breaker in installation B is open. The very small line-earth capacitance of the short connection (only a few metres of cable or bar) together with the inductance of the reactor leads to a high-frequency inrush voltage which can reach unduly high amplitudes. Hence in order to avoid disruptive discharges at the reactor terminals or at the open end of the “short” cable or bar connection surge arresters or limiters have to reduce the overvoltage to permissible values.
In contrast to this, when the breaker in installation B closes first, no significant overvoltages occur at the “long” end in installation A, as the closing overvoltage at the open circuit-breaker remains small due to the much higher earth capacitance of the long cable.
Current: 0 A; only on closing under no-load condition cos ϕ: – – –
Remarks: The energising of the cable at the “long” end can cause overvoltages at the “short”, open end.
It is recommended to install surge arresters.
Table B-6: Switching of a line or cable connection with reactor
Surge arresters should be installed in the station with the “short” cable or bar connection between reactor and circuit-breaker (installation B). The arresters can be mounted either at the terminals of the breaker or at the reactor.
Figure B-2: Surge arresters in line-earth connection at the circuit-breaker (preferably) or at the reactor
Surge arresters for installation at … Operating voltage 5
of system up to: circuit-breaker short-circuit limiting reactor
3.6 kV 3EF1 036-0A
4.8 kV 3EF1 048-0A
7.2 kV 3EF1 072-0A
12 kV 3EF1 120-0A
15 kV 3EF1 150-0A
Indoor: type 3EF1,
same type as at the circuit-breaker. Outdoor: arresters with equal protection level; selection see chapter E.
> 15 kV Selection see chapter E
Table B-7: Recommended surge arresters for the reactor
5 For other voltages and those above 24 kV any surge arrester can be selected; selection criteria are described in section E.
B 7
Compensation coils (shunt reactors)
These compensate the capacitive charging current on unloaded networks. Compensation coils are switched daily and thus the circuit-breakers reach large numbers of operations. The high rate of rise of the recovery voltage makes this a very difficult duty for conventional units, whereas vacuum breakers master these conditions excellently.
Current: up to 2000 A cos φ: 0.15 ind.
Remarks: - RC-circuits individually matched to the network are generally required
- surge arresters are required if the coil current is ≤ 600 A
Table B-8: Switching of compensation coils
Compensation coils are always equipped with an RC-circuit which prevents resonant harmonics in the coil during switch-off. When the coil current is ≤ 600 A, surge arresters are also fitted to prevent overvoltages.
In case of certain network conditions, where the line-earth capacitance on the feeding side is very small, additional protection at the busbar (x) by RC-circuitry and surge arresters is required.
Coil current I Protection I ≤ 600 A RC-circuits
+ arresters
I > 600 A RC- circuits
Figure B-3: Surge protection for compensating coils
The protection devices must always be individually matched to the network characteristics and data of the equipment.
B 8
Motors
This area of application covers many types of machines:
- Asynchronous motor: cage rotor, slipring rotor - Synchronous motor (with asynchronous start) - Motor with unit transformer
- Motor with starting transformer
- Motor with starting converter (“soft starter”) - Motor with individual power factor compensation
Duty: Motor during starting
Machine with locked rotor Normal operation, no-load up to full load Current: Cage rotor 5 … 7 ⋅ IrM
Slip ring rotor 1 … 2 ⋅ IrM
0.1 … 1.2 ⋅ IrM
cos φ: 0.2 to 0.3 inductive 0.9 to 0.3 inductive Switching of starting currents Ist ≤ 600 A 6
can cause overvoltages as result of multi-ple re-ignition and virtual current chopping
Switching of normal operating currents causes no impermissible overvoltages.
Remarks:
Motors with starting current ≤ 600 A6 must be equipped with surge arresters; except motors with individual power factor correction.
Table B-9: Switching of motors
B 8.1
Overvoltage protection for motors
For economical reasons the insulation level of machine windings, particularly the impulse withstand capability, is considerably lower than the insulation level of switchgear; see table 1 in [2]. This does not play any role when switching under normal service conditions. However, when a motor is switched off with blocked rotor or during start-up, inadmissibly high overvoltages can occur which must be limited by surge arresters. Exceptions are motors with power-factor correction, which do not need overvoltage protection, if the compensation power amounts at least 1/5 of the motor appar-ent power, see chapter B 8.6.
A simple decision criterion, whether overvoltage protection is required or not, is the starting cur-rent. Is the latter in the area up to 600 A, the motor must be equipped with overvoltage protection. For motor-transformer combinations, the current flowing through the circuit-breaker (or contactor) is the decisive criterion. Further considerations of other installation parameters, such as e.g. the ca-ble to the motor, thus are no longer required; [3], [4].
The following clauses show various arrangements with different connections points of surge limit-ers or surge arrestlimit-ers.
B 8.2
Direct connected motor (direct on line)
M
Ist ≤ 600 A Surge limiters inphase-earth connection at the circuit-breaker / contactor
Figure B-4: Surge arresters at the breaker terminals
At first sight, surge arresters at the breaker or contactor contradict the known rule to connect arrest-ers at the object to be protected. However, there are good reasons for it: arrestarrest-ers are susceptible against vibration and high ambient temperatures prevailing in a motor terminal box. Moreover a large terminal box would be required for which most often there is no space. Surge arresters at the breaker / contactor must have a particularly low residual voltage so that they protect the motor de-spite the travelling wave effects at the cable end on the motor side. The Siemens surge limiters type 3 EF or other arresters with the same protection level are recommended (Chapter E). 3 EF type is an especially developed arrester for the protection of motors or other equipment with sensitive wind-ings [5]. It satisfies the prerequisite of a particularly low residual voltage and therefore can be at-tached also at the breaker side of the motor cable.
Motor voltage up to:
3.6 kV 4.8 kV 7.2 kV 12 kV 7 15 kV
Surge limiter
types 3EF1 036-0A 3EF1 048-0A 3EF1 072-0A 3EF3 120-1 3EF1 150-0A
Table B-10: Recommended surge limiters for motors
B 8.3
Motor with transformer (unit)
Two variants of overvoltage protection are possible for motors with block transformer at the same protective effect.
Arrangement 1
Surge limiters in phase-earth connection at the cir-cuit-breaker or contactor
Arrangement 2
(prefered)
Surge limiters or arrester in phase-earth connection at the transformer
Figure B-5: Surge protetion for a motor-transformer combination
Recommended surge arresters are the Siemens 3EF surge limiters or other arresters with equal protection level; see chapter E.
7 For direct connected motors with 12 kV surge limiters at the breaker the type underlined must be used to ensure the protection level required.
Surge arresters for installation at the Rated voltage of the motor
or transformer up to: circuit-breaker transformer
3.6 kV 3EF1 036-0A 3EF1 036-0
4.8 kV 3EF1 048-0A 3EF1 048-0
7.2 kV 3EF1 072-0A 3EF1 072-0
12 kV 3EF1 120-0A 3EF1 120-0A
15 kV 3EF1 150-0A 3EF1 150-0A
Table B-11: Recommended surge limiters for motor-transformer combinations
Note to arrangement 2: At the transformer normal arresters can also be used (see chapters E) instead of the recommended surge limiters if the transformer insulation corresponds to the upper value of the rated insulation levels to table 2, IEC 60076-3 [6]; the values are the same as in column (4), Table H-3.
B 8.4
Motor with starting transformer
Starting current Connection of surge limiters RC circuitry Ist ≤ 600 A at main main switch (1) or transformer (2);
and at transformer neutral point (3) Ist > 600 A at transformer neutral point (3)
always at transformer neutral point, irrespective of the starting current
The insulation level of starting transformers8 frequently does not meet the values standardised for power transformers according to table 2, IEC 60076-3 [6]. Therefore, only surge limiters 3EF should be installed at the transformer (2), because of their very low residual voltage (standard surge arresters may not be suitable for this duty). Recommended devices are the Siemens 3EF surge limit-ers or other surge arrestlimit-ers at equal protection level. The arrestlimit-ers at the transformer neutral point must be selected in accordance with rated voltage of the transformer and the insulation level of its neutral point. Additionally, RC- circuitry must be installed at the transformer neutral point, as listed in Table B-12. Surge arresters at … Rated voltage of transformer up to: circuit-breaker transformer RC circuitry
3.6 kV 3EF1 036-0A 3EF1 036-0A
4.8 kV 3EF1 048-0A 3EF1 048-0A
7.2 kV 3EF1 072-0A 3EF1 072-0A
12 kV 3EF3 120-1 3EF1 120-0A
15 kV 3EF1 150-0A 3EF1 150-0A
R = 30 Ohms C = 200 nF
The values for R and C are valid with a cable length lC < 30 m between transformer neutral and circuit-breaker.
Table B-12: Recommended surge limites for motors with starting transformer
B 8.5
Motors with starting converter
1a 1b
2
3a 3b Surge limiters in line earth connection at the three con-nection points 1a or 1b, 2 and 3a or 3b, if:
• Circuit-breakers S1 to S3 are not interlocked with the converter, and
• The motor starting current is ≤ 600 A.
Figure B-7: Motor with starting converter (example with two transformers)
8 Autotransformers have a gapped iron core to increase the magnetizing current during the reactor starting phase (i.e. second phase of start-up with open transformer neutral). Transformers with a closed iron core are not suitable for this starting method.
Starting converters (“soft starters”) are bridged or switched off after the start-up of the motor. Under normal service conditions the converter controls the current and the breaking operation. However, instantaneous tripping of the circuit-breaker during the start-up phase cannot be precluded unless the control of the converter is interlocked with the circuit-breaker (the circuit-breaker must be fitted with a leading auxiliary switch for this purpose).
• Surge protection is not required if the converter control and the corresponding interlockings
en-sure that the circuit-breakers (1) to (3) never switch off starting currents ≤ 600 A.
• If the circuit-breakers are not included in the control of the converter and the motor starting
cur-rent is ≤ 600 A, surge protection must be installed for the converter transformers and the motor. The surge limiters for the motor must be mounted at the circuit-breaker S2 (connection point 2). For the converter transformers they may be mounted at the circuit-breaker (connection points 1a and 3a) or alternatively at the transformer terminals (connection points 1b and 3b). Surge limiters according to chapter B 8.2 are applied to the motor; for the converter transformers refer to chap-ter B 11.2 and B 11.3.
B 8.6
Motors with their own p.f. improvement
No overvoltage protection is necessary.
Motors with individual power factor correction do not need overvoltage protection, if the p.f. cor-rection capacitors are permanently connected to the motor and the compensation rating (QC) is at least 1/5 of the motor's rated power (SrM), normally QC = (1/3) ⋅ SrM is installed. The compensation capacitance reduces the frequency of the transient recovery voltage for the first pole-to-clear to less than the limit where multiple reignitions occur. Therfore no overvoltages occur either.
Thus individual compensation can also be used as an alternative method of providing protection.
Prerequisite: QC ≥ ⋅SrM 5 1 normally QC = ⋅SrM 3 1
B 9
Generators
Duty Generator short-circuited Load and no-load operation
Current: short-circuit current9 up to 1.2 ⋅ IrG
cos φ: 0.15 lag. lag. ↔ lead.
When short-circuit currents Ik" ≤ 600 A 10 are switched off, overvoltages due to multiple re-ignitions may occur.
Switching off under load or no-load conditions causes no im-permissible overvoltages Remarks:
In many cases surge protection is basically installed to ensure safety of operation since overvoltages may be transferred from elsewhere in the network.
Table B-13: Switching of generators
B 9.1
Selection of the circuit-breaker
Special attention has to be paid to the rated voltage and rated short-circuit breaking current when selecting a breaker for generators.
Rated voltage: The event of load shedding has to be considered when selecting this rating. The volt-age on the generator side of the breaker rises after load shedding. A standard value of 20 % rise is usually assumed. A higher value up to 40 % may be assumed if the simultaneous occurrence of sev-eral faults need be considered (e.g. load rejection and defective voltage regulator). The requirements for the circuit-breaker should be co-ordinated with the client.
Rated short-circuit breaking current: The peak value (making current) and the DC component of the current during short-circuit near-to-generator11 are higher than during short-circuit elsewhere in the system (“far-from-generator”). Typically the DC component is very high so that in some cases the AC component may decrease more rapidly than the DC component. This leads to delayed current zeros (DC component > 100 %). However the circuit-breaker needs current zeros for clearing. The real breaking current depends on the time interval between initiation of the short-circuit and the opening of the breaker because of the current decays during that time (tripping times of protection relay and opening time of c.b.). The values of the breaking current and the DC component must be stated by the customer or be calculated in case of doubt.
See also chapter H 2.3 for short-circuit current and DC component.
9 In case of fault at the busbar or breaker terminals
10 The current through the breaker is decisive in generator-transformer arrangements
11 The short-circuit is “near-to-generator” if the ratio initial to continuous symmetrical short-circuit current is I
B 9.2
Overvoltage protection
Switching of generators is similar to motor switching with respect to the transients. The decisive criterion for surge protection on account of the breaker is the short-circuit current of the generator during fault at the busbar or breaker terminals. If it is ≤ 600 A, surge limiters or arresters are used. However, surge protection is often universally fitted as a basic measure against overvoltages which may be carried over from the network.
B 9.3
Generators with I
k"
≤ 600 A feeding into a cable system
a) Direct connected generator
Surge arresters line-earth
Figure B-9: Surge protection for generators
Recommended surge arresters are the Siemens types 3EF and 3EE as well as other arresters with equal protection level.
Generator volt-age up to: 12
3.6 kV 4.8 kV 7.2 kV 12 kV 15 kV
Type 3EF 3EF1 036-0A 3EF3 036-0 3EF1 048-0A 3EF3 048-0 3EF1 072-0A 3EF3 072-0 3EF1 120-0A 3EF3 120-0 3EF1 150-0A
Type 3EE For the selection of ratings refer to chapter E
Table B-14: Recommended surge arresters or limiters for generators
For operating voltages above 15 kV the type 3EE2 is used; refer to chapter E for the selection of the ratings.
B 9.4
Generator-transformer unit
For generator-transformer combinations there are two arrangements of the surge arresters with equal effect of protection.
Arrangement 1
Surge arrester line-earth at the circuit-beaker
Arrangement 2 (preferred)
Surge arrester line-earth at the transformer
Figure B-10: Overvoltage protection of generator-transformer units
With arrangement 2 normal surge arresters can also be used at the transformer instead of the rec-ommended surge limiters if the transformer insulation complies with the upper value of the rated insulation levels to table 2, IEC 60076-3 [6]; the values are the same as in column (4),Table H-3.
Surge arresters at Transformer voltage13
up to:
circuit-breaker transformer
3.6 kV 3EF1 036-0A 3EF1 036-0A
4.8 kV 3EF1 048-0A 3EF1 048-0A
7.2 kV 3EF1 072-0A 3EF1 072-0A
12 kV 3EF3 120-1 3EF1 120-0A
15 kV 3EF1 150-0A 3EF1 150-0A
> 15 kV For the selection of ratings refer to chapter E
Table B-15: Recommended surge arresters for generator-transformer units
B 9.5
Generators connected to the HV grid
This almost only concerns generators of large size, which feed into high voltage systems ≥ 110 kV. The protective measures must be planned individually; they are independent of the breaker.
G Protective capacitors Machine transformer Station services transformer High voltage system ≥ 110 kV Generator circuit-breaker Machine arrester ~
Figure B-11: Generator connected to a HV grid
B 10
Furnace transformer
The following duties are included in this field:
- Arc furnaces, reduction furnaces, induction furnaces
- Exception: medium frequency furnaces are not included → see converter transformers.
Switching duty: Arc furnace Reduction furnace, induction furnace Current: 0.01 up to 2.0 x
rated transformer current 0.01 up to 2.0 x rated transformer current
cos φ 0.2 ←→ 0.9 lag. 0.5 ←→ 0.8 lag.
Remarks: During opening operations, multiple re-ignitions may cause resonant oscil-lations in the transformer; at currents ≤ 600 A high overvoltages due to virtual current chopping may occur as well.
Consider the insulation co-ordination; see section B 10.1
Table B-16: Switching of furnace transformers
Additional measures: RC-circuitry and surge arresters at the transformer and the busbar.
Circuit-breakers for furnace transformers have to meet very high electrical and mechanical require-ments partly under unfavourable operating conditions such as high temperature environrequire-ments or pollution from electrically conductive dust. The currents to be switched are between almost zero and two times the rated current of the furnace transformer; they may be unbalanced and distorted. The switching frequency may be up to 100 cycles per day and in exceptional cases it may be even higher.
B 10.1
Insulation co-ordination for operating voltages above 36 kV
Under certain conditions 36 kV vacuum circuit-breakers can also be used with higher operating voltages up to 40.5 kV (mainly in networks with effectively earthed neutral). Frequently a maxi-mum voltage of 38.5 kV is used which corresponds to installations with 35 kV, plus 10% tolerance. Insulation co-ordination: the insulation level of 36 kV circuit-breakers can be rated to maximum values 185 kV lightning impulse withstand voltage (BIL) and 85 kV short-duration
power-frequency withstand voltage (the standard insulation relating to the upper values according to IEC is 170 kV / 70 kV). All other equipment, including protective components such as RC-circuits and surge arresters, shall be correspondingly dimensioned; the complete switchgear installation must fulfil these insulation values.
B 10.2
Protective measures against overvoltage
Furnace transformers are generally equipped with RC-circuitry and surge arresters. RC elements prevent resonant oscillations in the transformer in that they form a high pass filter to earth for tran-sient high-frequency currents caused by multiple re-ignitions. Arresters limit the overvoltages caused by virtual current chopping, which can occur on switching off currents up to 600 A. Currents in this range are possible even if the rated operating current of the transformer is far above 600 A, as the actual values to be switched during operation lie between the magnetising current (no-load) of the furnace and twice the rated value (electrode short circuit). Depending on the method of neu-tral point earthing 3 surge arresters in phase-earth connection are used or additionally between the phases (6 arrester connection).
It depends on the network configuration whether the busbar needs surge protection as well. Installa-tions with a low earth capacitance of the inferring system require power capacitors at the busbar with a damping resistor (as shown in the figure below) or RC-circuitry similar to that on the trans-former. Under certain conditions surge arresters must be additionally fitted to busbar: they limit – together with the arresters at the transformer – the overvoltages across the contact gap.
Furnace transformer Arc furnace R C C-R circuitry at the busbar
Figure B-12: Overvoltage protection of a furnace transformer
The protection devices have to be matched individually to each system configuration by means of a system study. This optimises the efficiency of protection and is inevitably necessary to protect the components themselves. The arc furnace causes harmonics which in turn drive harmonic current through the capacitors. The latter plus the damping resistors must be dimensioned for the resulting thermal load. Furthermore the capacitance must be matched to the inductance of the inferring net-work in order to avoid resonances, which stress the equipment by voltage rise and reactive current. See also chapter I 2 for information.
B 11
Converter transformer
This heading comprises transformers with:
- controlled rectifiers - converter-fed drives
- voltage / frequency converters
- static compensators (static VAR or SVC)
Duty: unloaded loaded
Current: magnetising current 0.5 - 3 % rating current
up to rated current
cos φ: < 0.3 ind. lag. ←→ lead.
Note: See distribution transformers When switching off under load at currents ≤ 600 A, overvoltages may occur; there-fore overvoltage protection is required.
Table B-17: Switching of converter transformers
B 11.1
Protective measures
In normal service the load current is controlled by the converter so that the transformer is switched off only under no-load condition, which is uncritical – just like distribution transformers. However, a direct switching off operation, initiated by the circuit-breaker, cannot be precluded unless the con-trols of converter and circuit-breaker are interlocked (for this purpose the circuit-breaker must be fitted with a leading auxiliary switch). Switching off a loaded converter transformer may provoke the same conditions that prevail during the transformer inrush phase and can be associated with unduly high overvoltages. Therefore overvoltage protection should be generally installed. Experi-ence has revealed that in converter installations above all the line-to-line insulation must be pro-tected against overvoltages whereas the line-to-earth insulation is less critical.
In systems up to 15 kV operating voltage surge limiters or arresters in line-to-earth connection can also reduce the line-to-line switching surges to permissible values. This is, however, only applicable if the transformer is insulated in accordance with the upper rated insulation level of table 2, IEC 60076-3 [6] – these value are the same as of column (4), Table H-3.
All other cases require a 6-arrester arrangement which must be individually rated according to the operating voltage, method of system neutral point earthing and insulation level of th eequipment; see chapter E. The same applies to installations at operating voltages above 15 kV.
B 11.2
Systems up to 15 kV operating voltage
Figure B-13: Overvoltage protection of converter transformers up to 15 kV
Surge limiters or arresters at the circuit-breaker (1) are permissible only if the transformer insulation complies with the upper value of the rated insulation level according table 2, IEC 60076-3 [6]; the values correspond with column (4) Tabelle H2. If the insulation level is lower, the limiters or arrest-ers must be installed at the transformer (2).
B 11.3
Systems above 15 kV operating voltage
Figure B-14: Overvoltage protection of converter transformers above 15 kV
Suge limiters at Rated voltage of
trans-former up to: circuit-breaker transformer
3.6 kV 3EF1 036-0A 3EF1 036-0A
4.8 kV 3EF1 048-0A 3EF1 048-0A
7.2 kV 3EF1 072-0A 3EF1 072-0A
12 kV 3EF1 120-0A 3EF1 120-0A
15 kV 3EF1 150-0A 3EF1 150-0A
> 15 kV For the selection of ratings refer to chapter E
B 12
Capacitor banks and filter circuits
Capacitor banks and filter circuits pose similar demands on the circuit-breaker. Potentially critical impacts are the inrush current and the stress of the recovery voltage. The following paragraphs deal with the requirements of
- capacitor banks without reactors
- capacitor banks with inrush limiting reactors (reactor-capacitor units) - filter circuits
B 12.1
Capacitor banks (without reactor)
Duty: - Single capacitor bank
- Paralleling of capacitors (back-to-back switching)
Current: - up to 1.43 times the capacitor rated current at the fundamental component
(factor 1.43 includes harmonics and tolerances of the capacitance)
- on back-to-back switching, 100 times the rated current of the capacitor may
occur [9] cos φ: leading
Remarks: When paralleling, a high inrush current (Ie) with high rate of rise (considerably above the value of a short-circuit) may occur. The permissible inrush current (peak value) of a circuit-breaker depends on the geometry and the mechanical travel characteristics of the breaker contact and should not exceed following values:
- with flat contact pieces (vacuum circuit-breaker and contactor) Ie =10 kA - with tulip contact pieces (SF6-, minimum oil, compressed air) Ie = 5 kA When closing on a single capacitor bank, the inrush current does not exceed the peak value and the rate of rise of a power-frequency short-circuit, which the breaker must be capable to cope with in any case.
Measures: Circuit-breaker must feature a very low restrike probability and comply with class C 2 according to IEC 62271-100 [7].
Single capacitor banks do not require additional measures.
When back-to-back switching of capacitor banks, the inrush current must be determined and – where applicable – be limited; B 12.3.
Table B-19: Switching of capacitor banks (without reactor)
B 12.2
Reactor-capacitor combinations
Duty: - Single capacitor bank
- Paralleling of capacitors (back-to-back switching)
cos φ: leading
Remarks: After the opening operation the voltage on the capacitor side is higher than the system voltage (Figure B-18), so that the recovery voltage is higher in contrast to capacitors without reactor. The rated voltage of the breaker must not be exceeded by this effect.
When paralleling, high inrush currents (Ie) with high rate of rise may occur, depending on the reactor ratings. However, the inrush current does not exceed the permissible limits of breaker and capacitor if reactors are used (usually tuned to between 4th and 5th harmonic).
Measures: If the voltage at the breaker exceeds its rated value, either a breaker of a higher rated voltage level or 2 series-connected breakers have to be used.
Circuit-breaker must feature a very low restrike probability and comply with class C 2 according to IEC 62271-100 [7].
When back-to-back switching of capacitor banks, the inrush current must be determined and – where applicable – be limited; see B 12.3.
Table B-20: Switching of reactor-capacitor units and filters
B 12.3
Permissible inrush current
The permissible inrush current depends on the ratings of both the circuit-breaker and the capacitor bank.
Capacitor bank: Independent of the circuit-breaker, the peak value of the inrush current may not exceed 100 times the rated normal current of the capacitor, in order to limit the effect of the electro-dynamic forces. The factor 100 is a general rule only, at high switching rates the standard [9] rec-ommends to limit the inrush current to lower values.
Capacitors are normally equipped with discharge voltage transformers or resistors. If re-closing cycles may occur in normal service, the discharge time constant must be chosen short enough that the capacitor is almost completely (≤ 10 %) discharged before re-energising. Otherwise the inrush current can increase to undue values if the polarity of the system voltage is in opposition to the re-sidual charge.
Circuit-breaker: In order to avoid inadmissible stress and wear of the contact pieces, permissible limits of the inrush current must be observed. For Siemens circuit-breakers the following inrush currents are permissible without reservations:
- Ie ≤ 5 kA for tulip contacts (SF6 or minimum-oil circuit-breakers)
- Ie ≤ 10 kA for flat contacts (vacuum circuit-breakers)
Regarding flat contacts the limit value is founded on the tendency to contact welding if the inrush current does not decay rapidly enough during the pre-arcing time (1 … 2 ms) of the contact closing travel. Inrush currents above the limits mentioned above require an agreement with the manufac-turer. On the other hand, if the inrush current decays rapidly below the limit, considerably higher initial values of the inrush current are permissible (Figure B-15).
